An interest in energy storage technology has been increased. In particular, applications of electrochemical devices have been extended to portable telecommunication instruments such as compact mobile phones, camcorders and notebook computers, and further a power source for electric vehicles, and thereby efforts and attempts to research and develop such electrochemical devices are increasingly actualized. In this respect, the field of electrochemical energy storage devices has occupied a great deal of attention, and in particular, a lot of interest has been focused on the development of rechargeable secondary batteries.
Further, as a result of miniaturization and weight reduction of electric instruments, miniaturization and weight reduction of batteries used as a power source of the electronic instruments are also demanded. Therefore, recently, in order to increase capacity density of batteries, research and development for new designed batteries have been progressed.
Korean Patent Application No. 2001-5861 discloses a cell having a structure capable of maximizing content of electrode active material to prepare a small but high capacity cell, i.e., a stacked electrochemical cell having a structure including a plurality of stacked full cells or bicells and a separation film interposed between each stacked cell.
However, due to realization of high capacity of cells as described above, safety issues such as prevention of overcharge/over discharge and internal short-circuiting of cells have been increased. In particular, cell safety associated with overcharge of cells, hot box and nail penetration is a urgently resolved problem.
To improve the safety of cells, a method mounting a protective circuit and protective element on the cell or a method using heat blocking via a separator has been proposed. However, use of the protective circuit is greatly limited to realization of small and inexpensive battery packs, while a heat blocking mechanism by separator does not work effectively in many cases, when heat generation occurs abruptly.
Besides, a method using an organic electrolyte additive has been proposed, in order to resolve the problems associated with cell safety. For example, U.S. Pat. No. 6,074,776 discloses an example of preventing overcharge of cells by using a polymerizable monomer. In addition, Japanese Patent Laid-Open Publication No. 2000-215909 discloses an example of adding 1 to 10% by weight of branched polycyclic aromatic compounds and benzene compounds to an organic electrolyte solvent. However, use of the organic electrolyte additive may give rise to deterioration of cell performance.
Further, to prevent overcharge of cells, the different method is disclosed on Japanese Patent Laid-Open Publication No. 2000-164206. In accordance with this method, after carbon black as a conductive material and a binding agent are coated on a cathode current collector, then the mixed material of cathode active material and a binding agent are coated on that layer, such that when the cell is charged, resistance of the conductive material layer increases about 100 times, thus cutting-off current flow.
Alternatively, in order to improve cell safety, it was also disclosed to modify the surface of an electrode active material. Japanese Patent Laid-Open Publication No. Hei 9-55210 discloses a cathode active material prepared by coating lithium-nickel based oxides with alkoxides of Co, Al and Mn followed by heat treatment. Japanese Patent Laid-Open Publication No. Hei 11-16566 discloses lithium-based oxides coated with metals such as Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B or Mo and oxides thereof. And also, Japanese Patent Laid-Open Publication No. Hei 11-185758 discloses a cathode active material prepared by coating a surface of lithium-manganese oxides with metal oxides using a co-precipitation method, followed by heat treatment.
However, the above-mentioned methods have failed to elevate an initial temperature at which the surface of the electrode active material reacts with the electrolyte, i.e., a temperature (a heat generation temperature) at which oxygen molecules combined with metals of the cathode active material are liberated, on overcharging the cell, and also have failed to decrease a heat value.
In addition, U.S. Pat. No. 5,705,291 discloses a method for increasing structural stability of a cathode active material, by coating the surface of the cathode active material with a composition containing borate, aluminate, silicate, or mixtures thereof, but this method still exhibits poor structural stability.
Therefore, in the field of battery technology, there remains a need to develop the technology for improving the safety of cells.